Honeycomb with varying channel size

ABSTRACT

A honeycomb structure which includes an inlet end and an outlet end opposing each other and a plurality of cell channels extending along an axis from the inlet end to the outlet end, the cell channels having different hydraulic diameters and being arranged in a checkerboard pattern between large-diameter and small-diameter cell channels, and an extrusion die for making the same.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a multicellular structure, such as a honeycomb, particularly for trapping and combusting diesel exhaust particulates.

[0002] Wall-flow filters are used in the purification of diesel exhaust. Typically such diesel particulate filters are made of cordierite or silicon carbide and include a honeycomb body having thin interconnecting porous walls which form parallel cell channels of equal hydraulic diameter, longitudinally extending between the end faces of the structure. Alternating cells on one end face of the honeycomb are plugged with a ceramic filler material to form a “checkerboard” pattern. The pattern is reversed on the opposite side, so that the ends of each cell are blocked at only one end of the structure. When diesel exhaust enters the filter through one end face (i.e., inlet end), it is forced to pass through the porous walls in order to exit through the opposite end face (i.e., outlet end).

[0003] For diesel particulate filtration, honeycomb structures having cellular densities between about 10 and 300 cells/in² (about 1.5 to 46.5 cells/cm²), more typically between about 100 and 200 cells/in² (about 15.5 to 31 cells/cm²), are considered useful to provide sufficient thin wall surface area in a compact structure. Wall thickness can vary upwards from the minimum dimension providing structural integrity of about 0.002 in. (about 0.05 mm.), but are generally less than about 0.060 in. (1.5 mm.) to minimize filter volume. A range of between about 0.010 and 0.030 inches (about 0.25 and 0.76 mm.) e.g., 0.019 inches, is most often selected for these materials at the preferred cellular densities.

[0004] Interconnected open porosity of the thin walls may vary, but is most generally greater than about 25% of thin wall volume and usually greater than about 35% to allow fluid flow through the thin wall. Diesel filter integrity becomes questionable above about 70% open pore volume; volumes of about 50% are therefore typical. For diesel particulate filtration it is believed that the open porosity may be provided by pores in the channel walls having mean diameters in the range of about 1 to 60 microns, with a preferred range between about 10 and 50 microns.

[0005] Filtration efficiencies up to and in excess of 90% of the diesel exhaust particulates (by weight) can be achieved with the described cordierite materials. The filtration of a lesser but still significant portion (i.e. less than 50%) of the particulates may be desirable for other filtering applications including exhaust filtering of smaller diesel engines. Efficiencies, of course, will vary with the range and distribution of the size of the particulates carried within the exhaust stream. Volumetric porosity and mean pore size are typically specified as determined by conventional mercury-intrusion porosimetry.

[0006] U.S. Pat. No. 4,420,316 to Frost et al. discusses cordierite wall-flow diesel particulate filter designs. U.S. Pat. No. 5,914,187 discusses silicon carbide wall-flow diesel particulate filters.

[0007] There are significant problems associated with conventional filters of the type described herein. Specifically, as the exhaust passes through the filter, particulate matter (i.e., carbon soot) accumulates on the wall of the cell channels or in the pores of the wall and forms a soot layer. This soot layer decreases the hydraulic diameter of the cell channels causing a pressure drop across the length of the filter and a gradual rise in the back pressure of the filter against the engine, triggering the engine to work harder, and affective engine operating efficiency.

[0008] Eventually, the pressure drop becomes unacceptable which can be remedied by regeneration of the filter. In conventional systems, the regeneration process involves heating the filter to initiate combustion of the carbon soot layer. Normally, during regeneration the temperature in the filter rises from about 400-600° C. to a maximum of about 800-1000° C. Under certain circumstances, a so-called “uncontrolled regeneration” can occur when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas (such as engine idling conditions). During an uncontrolled regeneration, the combustion of the soot may produce temperature spikes within the filter which can thermally shock and crack, or even melt, the filter. The highest temperatures during regeneration tend to occur near the exit end of the filter due to the cumulative effects of the wave of soot combustion that progresses from the entrance face to the exit face of the filter as the exhaust flow carries the combustion heat down the filter.

[0009] In addition to capturing the carbon soot, the filter also traps metal oxide “ash” particles that are carried by the exhaust gas. These particles are not combustible and, therefore, are not removed during regeneration. However, if temperatures during uncontrolled regenerations are sufficiently high, the ash may eventually sinter to the filter or even react with the filter resulting in partial melting.

[0010] It would be considered an advancement in the art to obtain a diesel particulate filter which not only survives the numerous controlled regenerations over its lifetime, but also the much less frequent but more severe uncontrolled regenerations, while at the same time combining good fuel economy. This survival includes not only that the diesel particulate filter remains intact and continues to filter, but that the back pressure against the engine remains low.

SUMMARY OF THE INVENTION

[0011] The present invention provides porous ceramic honeycomb structures suitable for use in diesel particulate filters, the structures offering improved configurations that are significantly more resistant to thermal cracking and melting damage under typical diesel exhaust conditions than current designs. At the same time, the inventive structures offer significantly lower pressure drops across the filter and hence superior resistance to soot-induced engine back pressure build-up.

[0012] Temperature spikes which occur during regeneration, and especially during uncontrolled regeneration, are reduced in the inventive structures. At the same time, the inventive design provides structures with low initial filter pressure drop and a reduction in the system pressure drop during use.

[0013] In particular the invention relates to a honeycomb structure which includes an inlet end and an outlet end opposing each other and a plurality of cell channels extending along an axis from the inlet end to the outlet end, the cell channels having different hydraulic diameters and being arranged in a checkerboard pattern between large-diameter and small-diameter cell channels.

[0014] For a diesel particulate filter the large-diameter channels are inlet channels which are open at the inlet end and plugged at the outlet end. The small-diameter channels are outlet channels which are plugged at the inlet end and open at the outlet end. Each inlet channel adjoins an outlet channel in a vertical direction and in a horizontal direction, such that inlet channels and outlet channel alternate across the inlet and outlet ends. Preferably, the inlet cell channels have a hydraulic diameter of about 1.1-2.0 times, preferably 1.3 times larger than the outlet hydraulic diameter. For purposes of the present invention hydraulic diameter of the cell channels simply refers to the effective diameter of a cell channel.

[0015] The honeycomb structures may be formed of cordierite, silicon carbide or of other similarly porous but thermally durable ceramic material. Although the cell density is not critical in the present invention, it is preferred that the honeycomb structures have a cell channel density of about 100-300 cells/in² (15.5-46.5 cells/cm²), and more preferably about 200 cells/in² (15.5-31 cells/cm²) and a wall thickness about 0.01 to 0.25 inches (0.25-0.64 mm).

[0016] The invention also relates to an extrusion die for fabricating the inventive honeycomb structures. The novel die includes a die body which incorporates an inlet face, a discharge face opposite the inlet face, a plurality of feedholes extending from the inlet face into the body, and an intersecting array of discharge slots extending into the body from the discharge face to connect with the feed holes at feed hole/slot intersections within the die, the intersecting array of discharge slots being formed by side surfaces of a plurality of pins of two different cross sectional areas, alternating in size such as to form a checkerboard matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 illustrates a prior art end-plugged honeycomb structure with inlet and outlet cell channels of equal diameter;

[0018]FIG. 2 illustrates an embodiment of an end-plugged honeycomb structure according to the present invention with inlet cell channels of larger diameter than outlet cell channels; and,

[0019]FIG. 3 illustrates the effect of the modified cell geometry according to the present invention on the soot loaded pressure drop.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] Referring to FIG. 1 therein illustrated is a top view of a prior art end-plugged honeycomb structure with cell channels of equal hydraulic diameter. A top view of an embodiment of the present invention is illustrated in FIG. 2. Honeycomb 10 has a front or inlet end 12. Although not shown, the outlet end is opposite the inlet end 12. A plurality of cell channels which are divided into inlet cell channels 14 and outlet cell channels 16 extend between the inlet and outlet ends. The cell channels have porous walls 18 and run substantially longitudinal and mutually parallel between the inlet and outlet ends of the structure.

[0021] The cell channels are arranged to alternate between inlet cell channels 14 and outlet cell channels 16, resulting in a pattern of alternating cell channel with small and large hydraulic diameters. Therefore, each inlet cell channel 14 is bordered on all sides by outlet cell channels 16 and vice versa. The hydraulic diameter of inlet cell channels 14 is about 1.1-2.0 times, preferably about 1.3 times the hydraulic diameter of the outlet cell channels 16. Although not critical, preferably the structures have a cell density of about 100-300 cells/in² (15.5-46.5 cells/cm²), and more preferably about 200 cells/in² (15.5-31 cells/cm²), and preferably a wall thickness about 0.001 to 0.025 inches (0.25-0.64 mm), and more preferably about 0.019 inch (0.486 mm).

[0022] Both inlet cell channels 14 and outlet cell channels 16 are plugged along a portion of their lengths, either at the inlet end or the outlet end. In FIG. 2 since the inlet end is shown, outlet cell channels 16 are plugged. The plugs thickness is preferably about 2 mm to 5 mm in depth. Therefore, inlet cell channels 14 are open at the inlet end and plugged at the outlet end. Conversely, outlet cell channels 14 are plugged at the inlet end and open at the outlet end. This plugging checkerboard configuration allows more intimate contact between the fluid stream and the porous walls of the structure. Fluid stream flows into the honeycomb structure through inlet cell channels, then through the porous cell walls, and out of the structure through the outlet cell channels.

[0023] The inventive structures are especially suited as diesel particulate filters, especially in applications where regeneration of the filter by burning of the carbon soot can result in locally high temperatures within the filter, thus necessitating excellent thermal shock resistance and high melting point of the filter material. The inventive honeycomb structures may be formed of cordierite, silicon carbide or of other similarly porous but thermally durable ceramic material. The honeycomb structures may be either circular or square in cross-section. However, the filter of course need not have these shapes, but may be oval, rectangular, or any other cross-sectional shape that may be dictated by the particular exhaust system design selected for use.

[0024] An advantage of the present inventive filters is a low pressure drop across the length of the filter and therefore lower back pressure against the engine, resulting in better fuel efficiency during use. As it is known the pressure drop across an end-plugged honeycomb structure depends on the resistance to laminar flow of gas down the cell channels and, as a second order effect, the extend of gas contraction and expansion occurring as the gas traverses the cellular structure. Specifically, the pressure drop is directly related to the hydraulic diameter of the cells. In a conventional size cell channel as soot accumulates or builds up on the cell walls, the effective hydraulic diameter of a cell decreases resulting in an increase in the pressure drop.

[0025] However, in the inventive filters the inlet cell channels have a larger hydraulic diameter to start with and as such the effective hydraulic diameter after the soot accumulates is larger translating into a decrease in the rate of increase in the pressure drop across the length of the filter. This effect is presented in FIG. 3. 2″ diameter by 6″ long round filters were mounted in an airstream into which was added dispersed carbon black powder (Printex U brand). The filter was held in the airstream for a period of time for artificial soot build-up and then transferred to a pressure drop apparatus where the pressure drop was measured for a range of flowrates using air. After recording the pressure drop, the part was transferred back to the soot loading rig and additional soot was loaded into the filter. The part was again transferred to the pressure drop rig to measure pressure drop as a function of flowrate. This procedure was repeated several times and the data is plotted in FIG. 3 for a flowrate of 26.25 cfm. Pressure drop behavior is given for a conventional diesel particulate filter having a cell density of 200 cells/in² and a channel wall thickness of 0.019 inches (a so-called 200/19 honeycomb) and equal diameter cell channels, as well as for a diesel particulate filter made according to the present invention. The data confirm the benefit in soot loaded pressure drop for the structures of the present invention. It shows an approximate 25% lower pressure drop at higher soot loadings as compared with the conventional 200/19 geometry.

[0026] Another advantage of the present invention is expected to reside in a reduction in peak filter temperatures encountered during regeneration. For purposes of the present invention the term “peak filter temperature” refers to the maximum temperatures reached within the filter during regeneration. Specifically, the peak filter temperature is maintained at or below about 1050° C., in the inventive structures. Hence, the inventive filters are significantly more resistant to thermal cracking and melting than conventional cordierite filters under conditions encountered in diesel exhaust systems. This advantage is secured by an increase in the inlet open frontal area due to the larger diameter of the unplugged inlet cell channels at the inlet end. At the same time the open frontal area on the outlet end is decrease since the larger inlet cell channels are plugged and open are the smaller outlet cell channels. This has the effect of increasing the thermal mass of the filter at the outlet end, and securing resistance of the filter to temperature increase from regeneration. Therefore, the inventive filters have a large inlet open frontal area which maximizes the surface area for low pressure drop and increased thermal mass from the small open outlet cell channels at the outlet end.

[0027] The invention also relates to an extrusion die for fabricating the inventive honeycomb structures. Honeycomb extrusion dies suitable for the manufacture of honeycomb bodies with alternating channel diameters, as described in the present invention, will have pin arrays comprising pins of alternating size. It is not critical to the present invention how such dies are fabricated and as such could be provided by any one of a number of known methods, including the assembly of arrays of plates as disclosed in U.S. Pat. No. 4,468,365 or by bonding pin arrays to a suitable die base plate as described in U.S. Pat. No. 5,761,787. A preferred method however would be to use a plunge EDM process with an EDM electrode configured to have multiple rows of parallel-aligned tabs of two different cross-sectional areas alternatively arranged.

[0028] Therefore, the novel die includes a die body which incorporates an inlet face, a discharge face opposite the inlet face, a plurality of feedholes extending from the inlet face into the body, and an intersecting array of discharge slots extending into the body from the discharge face to connect with the feed holes at feed hole/slot intersections within the die, the intersecting array of discharge slots being formed by side surfaces of a plurality of pins of two different cross sectional areas, alternating in size such as to form a checkerboard matrix.

[0029] A suitable method for fabricating the inventive structures is by forming a plasticized mixture of powdered raw materials which is then extruded through into a honeycomb body with alternating cell channel diameters, optionally dried and then fired to form the product structure. The fired honeycomb filter is typically mounted by positioning the filter snugly within a cylindrical steel filter enclosure with a refractory resilient mat disposed between the filter sidewall and the wall of the enclosure. The ends of the enclosure may then be provided with inlet and outlet cones for channeling exhaust gas into and through the alternately plugged channels and porous wall of the structure.

[0030] A still further advantage of the inventive filter design is that the plugging process is made easier due to the differences in size of the inlet and outlet cell channels. Typically the plugging process is carried out manually and involves the employment of a flexible mask in one of the filter to cover every other cell channel in a checkerboard pattern. The exposed channels are then filled with a ceramic paste (of a material similar the honeycomb structure) that can be fired to result in a ceramic plug. The pattern is reverse on the opposite end of the structure to plug each cell channel only at one end. A disadvantage of this procedure is that after plugging the first side, it is difficult to determine which holes to insert mask into on the second end. In a preferred embodiment, separate masks are fitted to the front or inlet end and to the back or outlet end to overcome the aforementioned disadvantage. The honeycomb structures may be plugged either before or after firing with a paste having the same or similar composition to that of the green body, using appropriate amounts of a liquid phase to impart a workable viscosity, optionally with the addition of binders and plasticizers, as described in U.S. Pat. No. 4,329,162.

[0031] In addition to the embodiments presented herewith, persons skilled in the art can see that numerous modifications and changes may be made to the above invention without departing from the intended spirit and scope thereof. 

What is claimed is:
 1. A honeycomb structure comprising: an inlet end and an outlet end opposing each other; a plurality of cell channels extending along an axis from the inlet end to the outlet end, wherein the cell channels have different hydraulic diameters and are arranged in a checkerboard pattern between large-diameter and small-diameter cell channels.
 2. The honeycomb in accordance with claim 1 wherein the large channels have a hydraulic diameter 1.1-2 times larger than the small channels.
 3. The honeycomb in accordance with claim 2 wherein the large channels have a hydraulic diameter 1.3 times larger than the small channels.
 4. A ceramic filter for trapping and combusting diesel exhaust particulates, the filter comprising a honeycomb body, the honeycomb body comprising: an inlet end; an outlet end opposite the inlet end; a plurality of parallel cell channels extending in an axial direction between the inlet and outlet end, the plurality of cell channels having thin porous walls, the plurality of cell channels comprising: a group of outlet cell channels each end-plugged at the inlet end and open and the outlet end; a group of inlet cell channels each open at the inlet end and end-plugged at the outlet end, the inlet channels having a larger hydraulic diameter than the outlet channels; wherein each inlet channel adjoins an outlet channel in a vertical direction and in a horizontal direction, such that inlet channels and outlet channel alternate across the honeycomb body.
 5. A ceramic filter in accordance with claim 4 wherein the inlet channels have a hydraulic diameter 1.1-2 times larger than the outlet channels.
 6. A ceramic filter in accordance with claim 5 wherein the inlet channels have a hydraulic diameter 1.3 times larger than the outlet channels.
 7. A ceramic filter in accordance with claim 5 which is made of cordierite.
 8. A ceramic filter in accordance with claim 5 which is made of silicon carbide.
 9. A method of removing particulates from an exhaust gas, the method comprising passing the exhaust gas through the open inlet cell channels at the inlet end of filter of claim 4, through the cell walls, and out of the honeycomb through the open outlet cell channels at the outlet end, whereby the particulates are removed from the exhaust gas and retained on the honeycomb filter.
 10. A honeycomb extrusion die comprising a die body, the die body comprising: an inlet face, a discharge face opposite the inlet face, a plurality of feedholes extending from the inlet face into the body, an intersecting array of discharge slots extending into the body from the discharge face to connect with the feed holes at feed hole/slot intersections within the die, the intersecting array of discharge slots being formed by side surfaces of a plurality of pins of two different cross sectional areas forming a checkerboard matrix of pins alternating in size.
 11. The honeycomb extrusion die according to claim 10 wherein pins have a square cross section. 